1. Field of the Invention
The present invention relates to polyisocyanate-modified dicarboxylic acid derivatives, to a process for preparing these derivatives and to their use as curing agents for resins containing epoxy groups.
2. Description of the Prior Art
It is known to react dicarboxylic acids with polyepoxy compounds with opening of the epoxy ring to form high molecular weight polymers. The properties of coatings produced from these resins, such as impact strength and flexibility, are not entirely satisfactory for certain applications, for example, in automotive topcoat lacquers.
To reduce emissions and also fire and explosion hazards, particular emphasis has been placed on replacing systems containing solvents with powder coatings. In this case too, coating properties do not always satisfy the customer's requirements. The coatings often do not exhibit the desired resistance to chemicals and solvents. This is generally attributable to an excessively low crosslinking density.
An object of the present invention is to provide coating compositions, in particular powder coating compositions, having improved resistance to chemicals and solvents combined with good impact strength.
This object may be achieved with the curing agents for carboxyl/epoxy lacquers, in particular powder coatings, described hereinafter, which may be used to produce coatings having improved resistance to chemicals and solvents.
U.S. Pat. No. 5,371,167 describes compounds containing carboxylic acid groups as curing agents for carboxyl/epoxy powder coatings. These curing agents provide higher crosslinking densities and are produced from hydroxy- or aminocarboxylic acids and polyisocyanates, in which the hydroxyl or amino group is reacted with the isocyanate group. Particular care must be taken to ensure that during this synthesis the dual-functional hydroxy- or aminocarboxylic acid is used to the extent possible in strictly stoichiometric quantities. Any excess of hydroxycarboxylic acid or aminocarboxylic acid in the crosslinking agent reacts with (destroys) epoxy groups in the polyepoxy resin to be crosslinked without forming any crosslinks. An excess of polyisocyanate, in particular those having a functionality of &gt;2, reacts with the polycarboxyl compounds to form crosslinked or high molecular weight polymers, which are disadvantageous for use as a curing agent for epoxy resins due to their inability to form a defect-free coating surface.
It is known that amides and ureas are formed when isocyanates react with carboxylic acids with the liberation of carbon dioxide. It is known that the reaction of polyisocyanates having a functionality of .gtoreq.2 with dicarboxylic acids having a carboxyl functionality of .gtoreq.2 to produce products containing carboxyl groups also gives rise, as the reaction ratio of the carboxyl and isocyanate groups approaches the equivalence point, to unwanted high molecular weight or crosslinked, infusible derivatives. In the presence of anhydride groups, additional crosslinking by imides, i.e., the reaction products of amides with anhydrides, is also expected.
EP-A 695,771 describes isocyanate-modified carboxylic anhydrides which are particularly suitable as powder coating curing agents. Additional crosslinking by reaction of the anhydride groups with the hydroxyl groups produced during the carboxyl/epoxy reaction is also possible with these polycarboxylic acids containing anhydride groups. One disadvantage is that addition of isocyanate to the curing agent, for example to increase toughness, increases the molecular weight to such an extent that flow problems may occur in the coating during application.
EP-A 695,771 also describes the addition of organic hydroxy-functional compounds to modify the curing agent and to control molecular weight. However, this consumes carboxyl or anhydride groups, which reduces the crosslinking potential of the curing agent. When organic hydroxyl compounds having a functionality of &gt;1 are used, relatively high molecular weight fractions may be formed or there may be a risk of crosslinking.
It has now surprisingly been found that, when polyisocyanates having a functionality of .gtoreq.2 are reacted with dicarboxylic acids containing anhydride groups, polycarboxylic acids which yield a low viscosity melt may be obtained by the addition of water. When these acids are combined with conventional binders containing epoxy groups, they allow for the production of high quality coating compositions. It was particularly surprising that crosslinking could be prevented in the hydrophobic reaction mixture by the addition of water.